Can Scoria be used in a Cob mix?

scoria as a thermal additive to cob

While researching how to make a cob oven 6-7 years ago, it took ages to come up with a final recipe that made sense to me. What I wanted was a cob oven mix that was cheap to construct. It had to be thermally superior to just straight cob, and the ingredients had to be locally available. That is when we discovered scoria.

Cob greatly benefits when scoria is introduced into Cob Oven design.

This article details why it is such an excellent component to use in your mix, and I’ll explain everything below.

What is scoria?

Scoria is a volcanic rock with a similar composition to basalt but differs because of the myriad tiny bubble-like formations or vesicles that it holds.

It is often mentioned alongside pumice but is much harder. The amount of surface area within a piece of scoria is massive because of its vesicular nature. This has led to landscapers, aquaponic operators, and gardeners taking up the rock as a useful product.

It is also used as a concrete component in some areas, with the tested strength being above conventional concrete when up to 20% scoria is added.

Are there benefits of using scoria in cob?

The main benefits of using scoria in a cob oven mix are superior thermal qualities and a harder cob mix when dry.

As noted above, scoria is full of tiny vesicles or bubbles that are formed through volcanic action. It is also quite hard.

These two attributes lend it to be used in processes like cob, where the thermal qualities derived from the bubbles, along with the volcanic heritage and heat resistance it offers, are beneficial.

Being derived from basaltic mineral blends gives the rock a true hardness that helps reinforce the oven wall simultaneously.

When designing a cob oven, as you explore the available information on the internet, you will note that a common desire is for the oven to last a long time and retain as much heat as possible.

Both qualities are met by scoria, so if it is available to you, we recommend it. We also suggest considering what oven will suit your purposes if you wish to build one.

The design choice of your oven might be either a standard cob oven or a rocket stove cob oven that you see on this post. To help you understand the different attributes of them both, we have an article titled “cob oven vs. rocket stove cob oven, what best suits you?” It explains the different ovens in detail.

Scoria will take the heat as good, if not better than the clays most often used to bind the cob ingredients together.

scoria in a cob mix
Coconut husk is the straw substitute we used.

Pumice is sometimes mentioned, but it is far softer and more susceptible to decompose over time than scoria, so we wouldn’t recommend using it. Soft materials are something to avoid in a cob oven wall.

In certain areas of Cob Oven design, incorporating hard materials is a requirement if longevity is desired, and the internal floor of the oven is one such location.

Scoria, suited to heavier thermal insulation uses like cob oven construction, also introduces the required scuff resistance to the Cob Oven base. We have a post on using scoria in the oven base titled “how thick does a cob oven base need to be?” We recommend investigating if you are looking at the cob oven building process.

What to avoid when using scoria in cob

Avoid using too high a percentage of scoria to clay when using scoria in a cob mix. 20% scoria has been used in concrete, so without real-world data of Cob mixes that include scoria to reference, we suggest not exceeding this level.

Clay in the Cob mix will harden and last a very long time if the oven is not subjected to too much rain. If this is likely, cover the oven with a tarp or build a roof over it.

Too high a percentage of scoria can create a weak blend that has the potential to crumble over time. This will happen faster if the oven is exposed to the elements more than it should be.

A safety issue to be aware of when using scoria in a cob mix is the many sharp edges of this rock. Many people like to mix cob by walking the mixture together with bare feet.

If you are using scoria, many scratches and cuts will eventuate. The same can be said for hands when applying the cob to the walls during construction.

Gloves might be a good idea. We got around this issue by making cob bricks and letting them dry before laying them as part of the oven wall and roof.

Will cob last with scoria in it

If the percentage of scoria in the mix is at or below 20% of the cob blend, there is no reason the Cob Oven cannot last decades.

The amount of use should not significantly affect the oven’s life; we believe the more it is used, the better it becomes. This is what has happened with ours.

On a personal note, we used scoria in our cob oven when we built it around 2014-2015, and it has not missed a beat.

cob oven being built
scoria cob bricks being covered with more cob and straw.

There is very little damage from the heat, and the only repairs required have been from trials with securing the door, and door fitting.

Once the oven had dried completely, it operated as planned. It gets to temperature quickly, holds heat for as long as we want, and it was very cheap to build.

Alternatives to using scoria in cob

Alternatives to using scoria in a cob mix are small sharp gravels and coarse sand.

The small sharp gravels are suggested because the multifaceted angular shapes of each rock lend it to being able to be held by the clays from many different angles, and they lock into the cob wall once it has dried completely.

The same can be said for coarse sand. Both can be used in conjunction with more of the smaller sands in the mix. It is similar to a cement mix without cement powder.

The clay acts as a binder and a filler. The rocks reinforce, and the sand is a filler and binder. For more information on how we used scoria in our oven, we suggest this article. We also have an article on how to use a cob oven like ours for dehydrating here titled “using a cob oven,” and the final post we would suggest and one that probably would be a good one to start with is titled “how do you design a cob oven?

In closing, a Cob Oven is an excellent tool to have available. If the design is appropriate to your needs, the oven can play a part in a self-sufficient home.

Article by Tim Blanch for He is a qualified Permaculture designer.